SUBSIDENCE OF THE TEXAS COAST - INFERENCES FROM HISTORICAL AND LATE PLEISTOCENE SEA LEVELS

Changes in sea level observed at tide gauges are caused by actual chan ges in water level and by changes in elevation at the observing statio n. Recent research has focused on the relationship between climatic ch ange and sea level, but vertical land movement can be just as importan t, particularly in subsiding sedimentary basins. The purpose of this s tudy is to compare long-term rates of subsidence estimated from upper Pleistocene strata along the central Texas coast with historical subsi dence rates from the same area obtained from geodetic surveys and tide gauge data. This comparison shows that historical subsidence rates ar e much greater than long-term averages and are equal or greater than a ctual sea-level change along the Texas coast south of Galveston Bay. L ong-term (approximately 10(5) vr) subsidence rates were estimated by e stablishing the extent of marine, marine-influenced, and nonmarine str ata within the upper Pleistocene Beaumont Formation in the Copano Bay area of the central Texas coast, and comparing the maximum elevation o f in-place, marine-influenced deposits with published maximum sea leve l estimates of 5-8 m above mean sea level (MSL) from correlative, well -dated coral terraces from stable and uplifted areas. In-place, shell- bearing horizons deposited at or below sea level occur no higher than 2 m MSL in the Copano Bay area, suggesting that there has been no more than 6 m of subsidence since the probable time of deposition during t he Sangamon interglacial at approximately 120 ka. The long-term, avera ge subsidence rate for this part of the Texas coast is thus 0.05 mm/yr or less. Historical subsidence rates were obtained by: (1) calculatin g relative elevation changes between National Geodetic Survey first-or der leveling surveys conducted in the early 1950s with those conducted in the late 1970s to early 1980s; (2) normalizing the relative elevat ion differences between surveys to annual rates of change relative to an arbitrarily chosen benchmark; (3) referencing these lines to sea le vel at three tide gauges; and (4) comparing calculated rates of relati ve sea-level (RSL) rise along the lines with estimates of eustatic sea -level (ESL) rise. Rates of RSL rise for the Texas coast south of Galv eston Bay were generally 4-8 mm/yr; locally, rates were as high as 23 mm/yr. These rates are significantly higher than global averages of ap proximately 1 mm/yr. Much of the difference is probably caused by subs idence of the Texas coastal zone at rates of 1-22 mm/yr, or 20-440 tim es the long-term average of 0.05 mm/yr. The highest subsidence rates w ere found locally where there has been historical water-level decline in shallow aquifers. Lower subsidence rates of 3-7 mm/yr occur regiona lly where groundwater decline is minimal or nonexistent. Increased sub sidence over the long-term average in these areas may be caused by pre ssure decline in underlying oil and gas reservoirs.